The present invention generally relates to the structure and fabrication of Schottky diodes or rectifiers and, more particularly, to a dielectric isolation trench structure for such devices.
Various forms of Schottky rectifiers and their manners of operation are discussed in detail in U.S. Pat. No. 5,612,567 to B. J. BALIGA. Essentially, such rectifiers are fabricated from a first semiconductor N+ substrate layer having a metal cathode contact on one surface and a second epitaxially grown semiconductorxe2x80x94layer on its opposite surface. The secondxe2x80x94layer acts as the drift region and typically has discrete P+ regions located in its opposite surface in the drift region. A metal anode contact is formed on the opposite surface of the secondxe2x80x94layer and metal/semiconductor junctions form Schottky contact regions between the P+ regions. As noted in the Patent, the lowered operating voltages presently required for reduced power consumption and increased energy efficiency call for a decreased on-state voltage drop across the rectifier while maintaining high forward-biased current density levels and minimizing the reverse-biased leakage current. However, in Schottky barrier rectifiers there is a tradeoff between the forward-biased voltage drop and the reverse-biased leakage current so that it is difficult to minimize both at the same time. The doping level in the semiconductor region is used to affect the Schottky barrier height, but, while the use of a higher the doping level lowers the forward-biased voltage drop, it also lowers the reverse-biased breakdown level because of impact ionization.
Among the approaches for dealing with the problem of leakage currents, U.S. Pat. No. 5,365,102 is cited and discussed, for its disclosure of a trench MOS barrier Schottky (TMBS) rectifer wherein better than theoretically ideal breakdown voltage characteristics are achieved using a trench structure that causes the occurrence of charge coupling between majority charge carriers in mesa-shaped portions of the epitaxial/drift region of the trench structure and metal lining the insulated sidewalls of the trench. This charge coupling produces a redistribution of the electric field profile under the Schottky contact, which profile change results in achieving a breakdown voltage of about 25 Volts when an appropriate doping concentration for the drift region and selected oxide thickness are used. This compares quite favorably to the 9.5 Volts breakdown for an ideal abrupt parallel-plane rectifier. Additionally, because the peak electric field at the metal-semiconductor contact in TMBS rectifiers is reduced relative to an ideal rectifier, electric leakage current is also reduced. Contrary to the result obtained by increasing the depth of the trench, increasing the oxide thickness decreases the breakdown voltage. However, increasing the trench depth beyond a given depth will no longer increase the breakdown voltage beyond 25 Volts.
An earlier and apparently simpler approach is cited as being found in U.S. Pat. No. 4,982,260 to H-R. CHANG ET AL which describes the fabrication of a power rectifier with trenches, which rectifier is in the form of a p-i-n diode with Schottky contact regions. It is mentioned that Schottky diodes exhibit lower forward voltage drops and faster turn-off speeds than p-i-n diodes, this is at the expense of exhibiting high reverse leakage currents that increase significantly for increasing values of reverse voltage. Here, respective trenches are formed between the P+ regions extending into the drift region and containing the anode electrode conforming thereto with the Schottky contact region formed between a portion of the electrode at the lowest portion of each trench and the drift region. Field oxide layers are provided to partially overlie the extreme P+ regions at the edge of the active area of the rectifier for acting as a field plate for termination of the electric field generated by the device during operation.
The foregoing approaches for dealing with the leakage current problem in Schottky diodes and rectifiers, while offering their own advantages, still leave room for improvement in achieving an efficient and simplified rectifier structure that results in decreased on-state voltage drop across the rectifier while maintaining high forward-biased current density levels and minimizing the reverse-biased leakage current.
It is therefore an object of the present invention to provide a Schottky diode or rectifier having a structure that will operate to produce a decreased on-state voltage drop across the rectifier while maintaining high forward-biased current density levels and minimizing the reverse-biased leakage current.
In accordance with the present invention an improved diode or rectifier structure and method of fabrication is presented embodying the incorporation in a Schottky diode or rectifier, or the like, of an insulator or dielectric filled isolation trench structure formed in the epitaxial layer at the edge of the active area of the rectifier, for acting to enhance the field plate for termination of the electric field generated by the device during operation. The isolation trench is formed in a closed configuration about the drift region to more effectively terminate the electric field at the edge of the drift region. As a result the electric field is better concentrated within the drift region and acts to better interrupt reverse current flow and particularly restricts leakage current at the edges.